You are being advised a lot about good and bad prices and the range is pretty big. Let me try and put some context on this. In some parts of the country you may pay £15 per hour and in some you may pay £30 per hour or even more. This is influenced by many things not least the affluence of the area in which you live. I don't want to denigrate any part of the country more than any other but driving lessons in Newcastle aren't going to be the same price as driving lessons in Surrey. So comparing prices is a bit like comparing apples and pears - different things.
An instructor will generally charge what they think is reasonable in their area and the difference between low and high prices may only be £5 per hour. Instructors that are good are likely to charge more but not necessarily.
You also need to take into account that the price you'll pay in the first few lessons may be a specially discounted price because you are not going to be going far and therefore fuel cost is lower. The instructor may also be trying to 'hook' you in.
Finally, beware grades and pass rates. Grades may be 4,5,6 or A or B - they have changed the system relatively recently. Being a high grade does not necessarily mean a good instructor. It may just be that they are good at tests set by the DVSA. Likewise being a low grade may show inexperience (which doesn't necessarily mean bad) or someone who gets very nervous when being tested. Pass rate comparisons are only valid if they are calculated the same way and even then they can be misleading. If you give someone 150 lessons they are more likely to pass than someone who has had 15...